1. Field of the Invention
The present invention relates to soldering method of an electronic component of a soldered lead terminal plated with a lead-free metal, and a solder joint member.
Moreover, the present invention relates to a soldering method for soldering a printed wiring board including portions to be soldered in opposite surfaces, comprising: reflow-soldering one surface; and subsequently flow-soldering the other surface. The present invention particularly relates to a soldering method for performing a soldering operation in the above-described procedure, in which a fillet peel (circuit disconnection) is prevented from being generated in the reflow-soldered portion upon the flow-soldering.
Further, the present invention relates to a flow soldering method by a tin-zinc solder (solder mainly containing tin and zinc) which is superior in solder wetting/spreading and solder wetting/rising in a plated through hole even at a low temperature.
2. Description of the Related Art
(First Related Art)
As a substitute for plating by a lead-containing solder, a palladium or a palladium alloy layer has been formed on a lead frame surface of a semiconductor component, However, palladium has a problem that wettability is bad with respect to the tin-lead solder heretofore used, or potential candidates of a lead-free solder, such as a tin-silver-copper based solder and tin-copper based solder. There is also a lift-off problem that the solder floats from a copper land of the printed wiring board. Such related art and problems will be described hereinafter in detail.
As described above, as a technique for the lead-free electronic component, a method of forming the palladium or palladium alloy layer on the surface of the lead frame of the semiconductor component by plating has been performed. This is because palladium is superior in wettability with respect to a bonding wire of a wax material (silver wax) or gold during die bonding.
It is to be noted that rust is often generated in the lead frame only of the palladium or palladium alloy layer. Therefore, a nickel layer is generally formed as an underlayer,
FIG. 1 is a diagram showing a section of the lead frame (soldering lead) of the semiconductor component. For ease of seeing, a rather thick plated layer is shown.
That is, in a soldering lead 30, for example, the surface of a lead frame material such as an iron-nickel alloy 31 is once coated with the underlayer including a nickel layer 32, and further coated with an upper layer of a palladium layer 33 (or palladium alloy layer).
In order to mount electronic components such as the semiconductor component on the printed wiring board and actually solder the component by a flow soldering method, a method comprises: inserting the soldering lead terminal in the through hole of the printed wiring board; and bringing a soldering land formed around the through hole, and the soldering lead terminal into contact with a jet flow of a molten solder to solder the component. Under such a circumstance, a through hole is plated with copper to form the plated through hole in order to perform high-reliability soldering/mounting.
FIG. 2 is a longitudinal section view of the through hole, showing that the soldering lead terminal of the semiconductor component is inserted in the plated through hole of the printed wiring board.
Specifically, soldering lands 41 are disposed on the lower and upper surfaces of a printed wiring board 40, and these lands 41 are connected to each other via a plated through hole 42. A soldering lead terminal 43 of a semiconductor device is inserted in the through hole 42, and a portion to be soldered 44 is constituted between the lands 41 and plated through hole 42 and the soldering lead terminal 43.
In this condition, the jet flow of the molten solder is brought into contact with the lower surface of the printed wiring board 40 from below in FIG. 2, that is, the portion to be soldered 44 on the lower surface of the board to perform the flow soldering.
During the soldering/mounting, it is necessary to supply the solder having a sufficiently molten state to the portion to be soldered 44 so that satisfactory wettability is obtained. In other words, this is necessary for satisfying satisfactory electric connection and sufficient mechanical strength/connection of the portion to be soldered 44. Moreover, the printed wiring board 40 including the plated through hole 42 must be sufficiently filled with the solder in the through hole 42 and be wetted up.
In the related art, the electronic component is soldered onto the printed wiring board, liquid flux is generally applied in order to secure soldering property. This liquid flux is applied by methods such as immersing, brushing, spraying, and foaming. A post flux for use in an electronic industry is generally constituted of a main component obtained by blending and dissolving rosin as a base resin and hydrohalide of amine as an activator in a lower alcohol-based organic solvent such as isopropyl alcohol.
However, since the temperature of the lead frame rises in a die bonding or wire bonding step during the manufacturing of the semiconductor component, even palladium or palladium alloy superior in a bonding property has a problem of deterioration of solder wettability of the lead frame during the soldering/mounting onto the printed wiring board. That is, the solder wettability of the soldering lead terminal of the semiconductor component is deteriorated.
FIG. 3 is a longitudinal section view of the through hole, showing a soldering state of the soldering lead terminal whose wettability is deteriorated.
Specifically, the deterioration of the wettability of a soldering lead terminal 53 prevents the wetting-up of the solder into a plated through hole 52, and a soldering land 51 disposed on the upper surface of a printed wiring board 50 is not sufficiently wetted up. Such wetting-up defect of a solder 54 is similarly generated in the tin-lead solder heretofore used, or the potential candidates of the lead-free solder such as the tin-silver-copper based solder and tin-copper based solder.
On the other hand, as the technique for the lead-free solder, the tin-silver-copper based solder is the potential candidate. This solder is used to prepare the soldering lead terminal of the electronic component subjected to various types of plating, and the terminal is inserted in the through hole plated with the pre-flux as the potential candidate for a printed wiring board surface treatment, which does not contain lead. When the tin-silver-copper based solder is used to solder the component, a disadvantage referred to as lift-off (bonding defect) is known to be generated. That is, the solder floats from the copper land of the printed wiring board. FIGS. 4A, 4B are bird's-eye views of the lift-off generated in the lead-free solder.
As described above, the solder in the molten state is not sufficiently filled or wetted into the plated through hole, or the lift-off occurs. Then, when the printed wiring board is mounted on the electronic apparatus and used, because of applied stresses (e.g., temperature change, vibration, acceleration, deformation by deflection of the printed wiring board, and the like) or changes with time, the soldering lands, that is, printed wirings disposed on the lower and upper surfaces of the printed wiring board are disconnected each other. Alternatively, there is a problem that the solder bonding is detached and the disconnection easily occurs. In other words, reliability of the electronic apparatus on which the printed wiring board is mounted is remarkably degraded.
(Second Related Art)
FIG. 5 is an explanatory view of an example of a soldering procedure in which one surface of a printed wiring board 60 is reflow-soldered and the other surface is flow-soldered by a related-art method. It is to be noted here that this related-art example shows an example of the printed wiring board 60 including a plated through hole 61 and encircled numerals 1 to 9 denote step numbers.
Specifically, at first in step 1, an adhesive 63 for bonding an electronic component 12 (e.g., SMD such as a chip component) to be flow-soldered is applied, and subsequently in step 2 the corresponding electronic component 12 is mounted. Thereafter, in step 3 the adhesive 63 applied in the step 1 is hardened, and the electronic component 12 is fixed onto a land 64 (circuit pattern).
Next in step 4, the printed wiring board 60 is reversed, and solders 65 such as cream solders are supplied beforehand onto the lands 64 on which soldering terminals of electronic components 62, 66 to be reflow-soldered are laid, Subsequently, in step 5, the electronic components 62, 66 such as the chip component and QFP IC are mounted.
Thereafter, in step 6, the reflow soldering is performed by a reflow soldering apparatus. Specifically, the solder 65 supplied in the step 4 is heated and molten, and portions to be soldered of the electronic components 62, 66 such as the chip component and QFP IC between soldering terminals 67 and lands 64 are soldered.
After the reflow soldering is completed, in step 7, for example, lead terminals 69 of an insertion type electronic component 68 (hereinafter referred to as an insertion component) such as a connector is inserted in the through hole 61 on the side of the reflow soldered surface of the printed wiring board. In step 8 the flow soldering is carried out by a flow soldering apparatus.
Specifically, the portion to be soldered formed between the land 64 and the electronic component 62 fixed on the land 64 in the step 3, and the portion to be soldered formed between the land 64 including the plated through hole 61 and the lead terminals 69 of the insertion component 68 inserted in the through hole 61 in the step 7 are brought into contact with the jet flow of the solder in the molten state. The solder is supplied to these portions to be soldered, and the flow soldering is performed.
Herein, it is general to apply the liquid flux in order to secure the soldering property upon the flow soldering of the electronic component 12 or insertion component 68 onto the printed wiring board 60. Such liquid flux is applied by the methods such as immersing, brushing, spraying, and foaming. The post flux for use in the electronic industry is generally constituted of the main component obtained by blending and dissolving rosin as the base resin and hydrohalide of amine as the activator in the lower alcohol-based organic solvent such as isopropyl alcohol.
By the above-described procedure, the printed wiring board 60 is cooled as in step 9 of FIG. 5, the soldering of the portions to be soldered in the opposite surfaces of the board is completed, and fillets 600 of the solder are formed on the portions to be soldered of the SMD (electronic component 12) and insertion component 68.
In this event, in case where the land 64 disposed on one surface of the printed wiring board 60 is connected to the land 64 disposed on the other surface via the through hole 61, in the flow soldering step 8, the solder 65 needs to wet up the inside of the plated through hole 61 and wet-spread over the opposite lands 64. Moreover, in case where the lead terminals 69 of the insertion component 68 are inserted in the plated through holes 61, as shown in the step 9, the solder 65 needs to wet-spread over the opposite lands 64 to form the fillets 600.
This prevents the plated through hole 61 from being cracked and causing circuit disconnection by stresses such as heat stress, acceleration, and vibration upon the mounting of the printed wiring board 60 onto the electronic apparatus. Therefore, reliability of the electronic apparatus operating by the printed wiring board 60 is prevented from being damaged.
Moreover, it is most important to prevent the solder of the portion reflow-soldered in the step 6 from being molten again in the step 8 of the flow soldering. That is, the temperature of the reflow-soldered portion rises by heat conducted from the jet flow of the solder which has been brought into contact with the printed wiring board 60 upon the flow soldering.
When the solder of the reflow-soldered portion is molten again during the flow soldering, the position of the reflow-soldered electronic component 62 moves during the flow soldering and a soldering defect is generated. In the extreme, the electronic component 62 is detached and moved from the land 64 constituting the portion to be soldered, and a circuit function defect is caused Even when the re-melting simply occurs, solder wet defect occurs and soldering strength drops.
To solve the problem, various soldering methods have been proposed.
For example, one example is described in Japanese Patent Application Laid-Open No. 2001-358456 (hereinafter referred to also as Patent Document 1). That is, in this related-art example, a tin-silver-bismuth based lead-free solder and tin-zinc-bismuth based lead-free solder are used. In this technique, “a melting range of each solder is allowed to shift and thereby disengagement or connection strength drop of the component connected by a second-surface reflow or first-surface reflow upon the flow soldering are suppressed” (see paragraph [0009] of the related-art example of Patent Document 1).
In the technique disclosed in the: related-art example, bismuth is mainly used as a melting point adjustment agent, and a bismuth content in the solder is controlled to adjust the melting range of the solder. However, when the solder contains bismuth, the solder of the portion to be soldered is easily embrittled. There is a problem that soldering strength rapidly and easily drops with respect to a cycle stress given to the portion to be soldered. In other words, there is no tenacity.
Further, in the related-art example, “phenomenon in which the component peels off from an electrode on a substrate together with the solder depending on the type of the component” (see paragraph [0005] of the related-art example) is described. However, a mechanism by which the phenomenon occurs is not clarified. Therefore, the problem is not solved by a theoretically appropriate technique. That is, bismuth is included in the lead-free solder, the melting point is thereby forced to drop, and an influence of temperature rise during the flow soldering is minimized if possible in the technique.
The present inventors have noted that “the phenomenon in which the component peels from the electrode on the substrate together with the solder”, that is, the fillet peel from the land occurs only with respect to the electronic component coated with the tin-lead solder or including, for example, the plated soldering terminal during the soldering using a tin-silver-x (another element) based lead-free solder, and have observed the peeled portion using an electronic microscope. As a result, segregation of lead has been recognized in the peeled portion. Specifically, it has been clarified that a micro ternary eutectic alloy composition of tin-silver-lead is formed in an interface of the land and fillet and the melting point is as low as 178° C. (see “Experimental Consideration concerning SMT Fillet Peel of Lead-Free Solder” Kazuhiko Tanabe, Yu Saito, Article Name: the 15th Electronics Mounting Academic Lecture General Assembly Collected Papers).
FIGS. 6A and 6B are diagrams showing an end surface of the portion to be soldered showing the fillet peel, FIG. 6A is an explanatory view of a normal soldered state, and FIG. 6B is an explanatory view of the state of the peeled fillets 600. In the drawing, a peeled portion 70 is shown in an exaggerated manner for ease of seeing. Thus, the fillet peel is generated in a peeled manner from the interface of the land 64 and solder.
It is to be noted that in the above-described related-art example “three-elements based solders such as Sn—Ag—Bi and Sn—Zn—Bi based solders are noted in a close-up manner as potential candidates of a Pb-free solder alloy replacing the Sn—Pb based solder”. “Sn-9 weight % Zn (melting point 199° C.) has a substantially appropriate melting point, but the solder surface is remarkably oxidized in the soldering in the atmosphere, and the solder is not easily used” (see paragraph [0003] of a first related-art example). This is set aside in a boundary point of the scope of the present invention.
Particularly, at a filing time of this related-art example, the flow soldering technique of the printed wiring board using the tin-9 zinc solder (tin-9 wt % zinc solder, the numeral before the element similarly denotes weight % hereinafter) had not been established yet. The flow soldering technique of the printed wiring board using the tin-9 zinc solder was thereafter published by Japanese Patent Application Laid-Open No. 2001-293559 as Patent Document 2, and technical establishment is known.
(Third Related Art)
Diffusion of lead toxicity into natural environments and influence thereof onto human bodies have raised problems, and the soldering of the electronic apparatus and printed wiring board has been performed by the lead-free solder without using the lead. However, most of presently used lead-free solders are lead-free solders, whose melting point is high at about 220° C. An optimum soldering temperature is higher than the soldering temperature (about 240° C. to 250° C.) of the tin-lead solder which has heretofore been used by about 10° C. to 15° C. and is in a range of about 250° C. to 255° C.
To flow-solder the printed wiring board on which the electronic component is mounted, namely, a work to be soldered, a heat stress is imposed onto the printed wiring board and electronic component. Specifically, it is necessary to bring the portion to be soldered, that is, the surface to be soldered of the printed wiring board into contact with the solder having the molten state and to solder the portion,
Moreover, the heat stress of the presently used lead-free solder having the high melting point becomes larger than before, and therefore a problem is that life of the electronic apparatus becomes shorter than before. As a countermeasure, development for enhancing heat resistance of the electronic component and development for realizing the flow soldering with a low-temperature solder have been performed.
The heat stress has a large influence on the life of the electronic component. Specifically, when the flow soldering is performed with the low-temperature solder, the electronic apparatus having a long life can be realized. Additionally, if the solder having an optimum soldering temperature lower than that of the tin-lead solder heretofore used is employed, the electronic apparatus having a longer life than before can be realized.
As described above, the temperature of the solder during the flow soldering appears as a difference of reliability which cannot apparently be judged from the electronic apparatus. Therefore, as an important index in supplying the high-reliability electronic component to a user, a flow soldering technique using the solder having a low optimum soldering temperature has been noted.
The melting point of the tin-zinc solder (e.g., the melting point of the tin-9 zinc solder) is about 199° C. and is low as compared with the melting point of another lead-free solder (e.g., about 220° C.). There is an advantage that the heat stress imposed onto the printed wiring board or the electronic component can be reduced during the flow soldering. However, it is regarded as impossible to flow-solder the printed wiring board using the tin-zinc solder, because soldering properties such as the wettability are not good.
Already in Japanese Patent Application No. 2002-4185, the present inventors have filed a patent application of a technique comprising: subjecting the land to be soldered or plated through hole of the printed wiring board to leveler treatments such as a hot air leveler (HAL) treatment by the tin-zinc solder to form a solder layer containing tin and zinc as main components on the surface of the board. Thereby, the electronic component in which the palladium or palladium alloy layer is formed on the surface of the soldering lead terminal of the electronic component can be soldered with superior wettability, and connection reliability of the portion to be soldered is also high.
Thereby, it has become possible to perform the flow soldering with the solder which has a temperature similar to or lower than the conventional temperature. The palladium or palladium alloy layer has been used for a lead-free member replacing the tin-zinc solder layer heretofore broadly used.
Herein, it is to be noted that the HAL treatment is a treatment comprising: immersing the printed wiring board into the solder or metal having the molten state; thereafter drawing the printed wiring board out of the solder in the molten state; and spraying air or inactive gas to the board to level the board.
For a printed wiring board on which a large number of various electronic components are mounted and which includes a large number of portions to be soldered, to collectively flow-solder the respective portions to be soldered, major factors for influencing the soldering properties such as the wettability and wet-up property of the solder are controlled. Thereby, the influence of other sub factors onto the soldering properties can be ignored. This enhances and uniforms soldering qualities of a large number of portions to be soldered. Additionally, this is necessary for enhancing and stabilizing the soldering qualities of the printed wiring boards produced in mass.
Therefore, in the flow soldering method using the tin-zinc solder, the present inventors have specified major factors in the printed wiring board, for enhancing the wettability and wet-up property of the tin-zinc solder in the land to be soldered and plated through hole in the printed wiring board. It has been regarded as a technical problem to clarify conditions for performing the flow soldering method using the tin-zinc solder with the stable soldering quality, and the flow soldering method has been developed.